This invention relates to binder resins for use in the preparation of toners for developing electrostatic images formed in the process of electrophotography, electrostatography, electrostatic recording or the like. More particularly, the invention relates to binder resins usable in preparing positively chargeable toners as well as negatively chargeable toners.
Electrostatic images are generally developed with a toner, which occurs as a powder, to give toner images. These toner images are fixed either as they are or after transfer to a receiving paper sheet or the like.
Generally, toners are prepared by admixing a binder component and a coloring component, as necessary together with a charge control agent, melting the mixture with heating to achieve homogeneous admixture and comminuting the resultant mass to a desired particle size.
The performance characteristics of a toner much depend on the binder component. A binder resin having an acid value can give negatively chargeable toners, whereas a binder resin having an amine or quaternary ammonium salt group as introduced therein can give positively chargeable toners.
Various resins are known to be useful as binders for toner preparation. Recently, polyester binders have attracted much attention since they have high levels of negative chargeability and good low-temperature fixability, hence are suited for use in high-speed copying. Furthermore, they have good resistance to plasticizers. In other words, they will not cause prints or copies carrying images developed therewith to become indistinct when said prints or copies are placed in contact with a polyvinyl chloride sheet or film, for instance, and accordingly exposed to a plasticizer contained in said sheet or film.
Polyesters described as being useful as toner binder resins include, among others, the following:
(i) Japanese Kokai Tokkyo Koho No. 59(1984)-7960 discloses a thermally fixable dry toner containing nonlinear, low melting-point polyesters with an acid value of 10-60 as obtained from components comprising (A) an alkyl-substituted dicarboxylic acid and/or an alkyl-substituted diol, (B) a polycarboxylic acid having three or more carboxyl groups and/or a polyol having three or more hydroxyl groups, (C) a dicarboxylic acid and (D) an etherified diphenol.
(ii) Japanese Patent Publication No. 52(1977)-25420 [Japanese Kokai Tokkyo Koho No. 47(1972)-12334]discloses a toner which comprises a polyester resin obtained from a dicarboxylic acid and a specific etherified diphenol and having a pour point lower than 110.degree. C. and a softening point of not lower than about 60.degree. C. The acid value of said resin is about 18-30, as described in the examples in the relevant specification.
(iii) Japanese Kokai Tokkyo Koho No. 59(1984)-29256 discloses a toner employing an amorphous polyester having an acid value of 10-100 as obtained by reacting an etherified diphenol propoxylated and/or ethoxylated with a propoxy group content of not less than 50% with a phthalic acid or acids containing up to 40 mole percent of an aromatic carboxylic acid having three or more carboxyl groups.
(iv) Japanese Kokai Tokkyo Koho No. 62(1987)-45622 discloses polyester resins for toner preparation which contain a nonvolatile monocarboxylic acid component, more specifically sulfobenzoic acid monoammonium salt, sulfobenzoic acid monosodium salt, cyclohexylaminocarbonylbenzoic acid, n-dodecylaminocarbonylbenzoic acid or the like, and have a glass transition point of 50.degree. C. and a softening point of 90-170.degree. C. Although no mention is made of the acid value and the hydroxyl value in the relevant specification, the product polyester must have a considerably great hydroxyl value since, in the examples described in said specification, the glycol component is used in considerable excess as compared with the acid component.
Among the above-mentioned polyester binder resins for toner preparation, those mentioned under (i), (ii) and (iii) are usable only for the preparation of negatively chargeable toners since they have a great acid value.
In the polyesters mentioned above under (iv), the excess hydroxyl groups are partly blocked by the benzoic acid type monocarboxylic acid. From the quantitative viewpoint, however, there must presumably exist a considerably large number of unblocked terminal hydroxyl groups. Investigations by the present inventors have revealed that hydroxyl-terminated polyesters have a difficult problem from the practical use viewpoint in that when positive charge control agents are added to toners based on such polyesters, only low levels of positive charge can be attained, although the addition of negative charge control agents can be expected to result in high levels of negative charging. Furthermore, toners based on the polyesters mentioned above under (iv) are not fully satisfactory in respect of low-temperature fixability and offset resistance. This offset phenomenon includes staining of the surface of the receiving sheet supplied next due to transfer of a part of toner to the heat roller and, further, staining of the back of the receiving sheet with that portion of toner further transferred to the pressure roller in contact with the heat roller under pressure.
Accordingly, it is a primary object of the invention to provide polyester binder resins which can be used in preparing both positively chargeable toners and negatively chargeable toners and can give toners having good low-temperature fixability and offset resistance.